Glaucoma
January 2010
by Tony Realini, M.D.
Is there new clinical utility to this old provocative test?
The water-drinking test was proposed decades ago as a means to diagnose glaucoma. The premise was straightforward. If glaucoma affects the drainage angle and impairs outflow facility, then the ability of eyes with glaucoma to handle a fluid challenge should be impaired. Following a baseline intraocular pressure (IOP) measurement, the subject quickly consumes one liter of water and undergoes subsequent IOP measurements every 15 minutes for one hour. Normal eyes should be able to handle the fluid challenge by increasing outflow, whereas a glaucomatous eye with impaired outflow would be less able to adapt to the fluid influx and thus should manifest an IOP rise. A rise in IOP of 6 to 8 mmHg or an increase from baseline of 30% or more at any time during the hour was considered a positive test.
“As a provocative test to diagnose glaucoma, the water-drinking test didn’t work,” said Remo Susanna Jr., M.D., University of São Paulo, Brazil. There were too many false-positive and false-negative outcomes to be useful.
“The water-drinking test should not be considered a provocative test for the diagnosis of glaucoma but as a stress test,” he said. “We can use the water-drinking test to perturb the aqueous fluid dynamic system, and in doing so, we can learn important things such as the extent to which IOP rises and the speed with which it recovers.”
New applications for an old test
Dr. Susanna illustrated the clinical utility of the water-drinking test by describing a series of research studies that he has conducted over the past several years. In one study, the water-drinking test was administered to subjects with bilateral primary open-angle glaucoma and symmetrical IOP levels but with markedly asymmetric visual field loss.
“The eye with worse glaucoma manifested a higher IOP rise than the better eye, and the worse eye also took longer to recover to pre-test IOP baseline than the better eye,” he said. He pointed out that the water-drinking test challenged the eye and revealed relevant data that was not apparent in the unperturbed system. “Intraocular pressure hemostasis was worse in the worse eye.”
But is this alteration in IOP regulation an underlying contributor to the glaucoma disease process or simply a manifestation of that process? To answer this question, Dr. Susanna described a retrospective longitudinal study in which the water-drinking test was administered to a cohort of subjects with open-angle glaucoma who were followed for two years. The cohort was then divided into two groups based on the presence or absence of visual field progression during the follow-up period.
“The IOP peaks in the hour following the water-drinking test were higher in the group that had visual field progression compared to the group whose visual fields had remained stable,” he said. This suggests that the water-drinking test can predict which patients with open-angle glaucoma may be at higher risk of progression, he added.
One of the more practical applications of the water-drinking test, he said, is to predict peak IOP in glaucomatous eyes. Mariluci Tosi, M.D., Brazil, agreed. “Peaks and fluctuations in IOP have been identified as important risk factors for glaucoma progression,” she said. “But detecting IOP peaks and fluctuation requires diurnal IOP measurement, which is difficult and expensive.”
“The water-drinking test predicts diurnal peak IOP,” said Dr. Susanna, basing his assertion on another study he conducted in which peak IOP following an in-office water-drinking test correlated very well with the peaks observed in diurnal curve testing of the same subjects.
Dr. Tosi has also conducted a research study which lends support to Dr. Susanna’s findings. In her study, 29 subjects underwent two separate water-drinking tests, one at 6 a.m. and one at 2 p.m., on the same day. “We determined the IOP peak as the highest IOP value during the one-hour period after the test, and the IOP fluctuation as the highest IOP value minus the lowest IOP value during the same period.”
Interestingly, while the mean baseline pre-test IOP was different for each water-drinking test trial, the mean peak IOPs achieved in the hour following each test were remarkably similar.
“In the morning, mean IOP rose from 14.3 mmHg to a peak of 17.3 mmHg, while in the afternoon, IOP rose from 12.7 mmHg to 16.8 mmHg,” Dr. Tosi said.
In terms of IOP fluctuation, she found that the median IOP amplitude was 5.0 mmHg in both groups.
She put these results into a clinical context. “Despite starting from different baseline IOP values, peak IOP values were the same in both tests, suggesting that the test evaluates the maximum capacity of the eye to reach its peak, independent of the baseline value.”
How it works
What is the mechanism by which the water-drinking test affects IOP? Dr. Susanna recently completed a study in which 30 subjects with open-angle glaucoma underwent A- and B-scan ultrasonography before and after the water-drinking test.
“There was a significant increase in choroidal thickness,” he said. He explained that consumption of water lowers blood colloid osmotic pressure, which can drive water from the systemic vasculature into the choroid along an osmotic gradient. Expansion of the choroid within the fixed volume of the globe will transiently raise IOP and simultaneously push fluid out through the drainage angle. “If outflow facility is impaired, as in glaucoma, IOP goes up even more and for a longer period of time,” Dr. Susanna said.
Editors’ note
Drs. Susanna and Tosi did not indicate any financial interests related to their comments.
Contact information
Susanna: rsusanna@terra.com.br
Tosi: minitosi@ig.com.br
